Method of producing supersaturated solutions of aluminum fluoride



United States Patent Int. Cl. c01r 7/50 US. CI. 23-88 6 Claims ABSTRACTOF THE DISCLOSURE supersaturated Solutions of aluminum fluoride areproduced with substantially 100% efliciency by reacting a solution ofhydrofluoric acid with an excess of aluminum oxide.

This invention relates to a method of producing supersaturated solutionsof aluminum fluoride.

As is known, aluminum fluoride, MP is produced conventionally byprecipitating its trihydrate, AlF -3H O, from a supersaturated AlFsolution. The supersaturated AlF solution is usually formed by reactingaluminum metal, aluminum hydroxide, or bauxite with fluorinecontainingmaterials such as HF, H SiF HBF etc. Attempts also have been made toform the supersaturated AlF solution by reacting aluminum oxide, A1 0with a dilute solution of HF. However, the efliciency and yield of thisprocess have proven to be very poor. In particular, reaction has beenattempted between stoichiometric amounts of A1 0 and HF as well as up to100% of excess HF, but only a maximum of 6% of the fluorine content ofthe HF could be converted to AlF by these methods.

The present invention is based upon a surprising and unexpecteddiscovery that reaction between Al O and dilute HF solutions may becarried out with substantially 100% efficiency and yield if asubstantial excess of A1 0 over the stoichiometric proportion is used.More specifi cally, it has been found possible to convert substantiallyall of the fluorine content of dilute HF solutions into a supersaturatedAlF solution if the HF solution is reacted with an amount of A1 0 whichis in excess by at least about 50% beyond the stoichiometric proportion.In terms of actual weight, the invention accordingly is based on thediscovery that substantially complete reaction may be achieved betweenA1 0 and HF solutions if the ratio of dry weight of HF to A1 0 is atleast about 1:1.275 as contrasted from the ratio of stoichiometricproportions which is 1:0.85.

The practical advantages given by the invention are particularlynoteworthy and valuable in connection with the conventional productionof aluminum by melt electrolysis. In aluminum plant operations, the offgases emanating from the electrolysis aluminum furnaces contain fluorinewhich is extracted by washing the off gases with water. The wash wateraccumulates from 3 to of HF and is often called tower acid. Whereasprior attempts to utilize the tower acid as raw material for productionof supersaturated AlF solutions by reaction with A1 0 have not beensuccessful, the total fluorine content of the tower acid may besubstantially completely recovered as AlF by reacting the tower acidwith an excess of A1 0 in accordance with the invention. Thus, aneconomic credit is achieved by the invention corresponding to the3,492,086 Patented Jan. 27, 1970 value of the fluorine contained in thetower acid. In addition, further savings are realized by the fact thatthe cost of aluminum in the form of aluminum oxide is lower than asaluminum hydroxide, and by the fact that all aluminum plants maintainlarge supply stocks of aluminum oxide at hand. Thus, the inventionpermits drawing upon the same supply stocks to recover the fluorinecontent of the tower acid without having to incur expenses for extrastorage units, hoppers and other supply equipment as would be requiredif a separate raw material were used for reaction with the tower acid.

Briefly stated, the process of the invention comprises reacting anaqueous solution of hydrofluoric acid with solid aluminum oxide in an HFto A1 0 dry weight ratio of at least about 1:1.275 to form asupersaturated solution of AlF The weight ratio specified above is aminimum which must be observed in order to form supersaturatedAllsolutions with the remarkable efliciency and yields provided by theinvention. However, this ratio may be increased to increase the degreeof excess of A1 0 over the stoichiometric proportion. Thus, the weightratio of HF to A1 0 may be increased up to about 1:12 and the conversionof substantially 100% of the initial HF solution to AlF in accordancewith the invention may be achieved with any weight ratio within therange from about 1:1.275 to about 1:12.

In carrying out the reaction, it is preferable to add the solid aluminumoxide in batches to the HF solution and to subject the mixture tovigorous agitation. Also, the speed of reaction can be increased byelevating the temperature to within the range from about to about 100 C.and this is generally desirable.

As previously noted, particular advantages are gained by utilizing toweracid containing 3 to 5% HF as the HF raw material and use of thismaterial is recommended for best results. However, the HF solution canbe derived from other sources, such as the pyrohydrolyticaldecomposition of aluminum furnace carbonaceous wastes, and theconcentration of HF in the solution may be high r, say, up to about 15%.After the supersaturated AlF solution has been formed, AlF may beprecipitated therefrom in an improved manner by use of aluminum oxide asprecipitation nuclei as is disclosed and claimed in the copendingapplication of Arnfinn Ve entitled Method of Producing AluminumFluoride, Ser. No. 608,984 filed Jan. 13, 1967.

Further details of the present invention will be apparent from thefollowing example which constitutes one specific embodiment thereof.

To 1,000 kilograms of a 5% aqueous HF solution were added 292.5kilograms of A1 0 in batches with vigorous stirring and heating to aboutC. Thus, the weight ratio of HF to A1 0 was 1:5.85 as contrasted fromthe ratio of stoichiometric proportions, 1:0.85. When the reactionbetween the HF and added A1 0 was completed, a suspension was formedcontaining 70 kilograms of dissolved AlF and 250 kilograms of suspendedA1 0 and this constituted a conversion of all of the original HF intoAlF The dissolved AlF was precipitated on the surface of the remainingA1 0 The resulting coprecipitate of Alland A1 0 was dried, calcined andrecycled as raw material to aluminum electrolytic furnaces.

It will be understood that it is intended to cover all changes andmodifications of the preferred embodiment of the invention, hereinchosen for the purpose of illustration, which do not constitutedepartures from the spirit and scope of the invention.

3 What is claimed is: 1. Method of producing supersaturated solutions ofaluminum fluoride which comprises reacting a hydrofluoric acid insolution with at least a 50% excess of aluminum oxide overstoichiometric proportions at a tem- 5 perature of at least about 75 C.but not greater than 100 C. to produce said supersaturated solution ofaluminum fluoride.

2. Method as in claim 1 wherein said hydrofluoric acid solution containsabout 3 to about 5% HF.

' 3. Method as in claim 2 wherein said hydrofluoric acid solution isformed by washing off gases from an electrolytic aluminum furnace withWater.

4. Method as in claim 1 wherein said aluminum oxide is added to saidhydrofluoric acid solution with vigorous agitation and elevation oftemperature to within the range from about 75 to about 100 C.

5. Method as in claim 1 wherein said aluminum oxide is added in batchesto said hydrofluoric acid solution.

'6. Method as in claim 1 wherein said hydrofluoric acid solutioncontains up to about 15% HF.

4 References Cited UNITED STATES PATENTS 2,842,426 7/1958 Glocker 23882,920,941 1/1960 Sanlaville et al. 23-154 3,057,681 10/1962 Gernes eta1. 23-88 FOREIGN PATENTS 735,156 8/1955 Great Britain.

OTHER REFERENCES McPherson & Henderson book A Course in GeneralChemistry, Third Ed. (1927), page 211, Pub. by Ginn 5 & Co., N.Y.

Hackhs Chemical Dictionary, Third Ed. Revised US. Cl. X.R. 23-301;204-67

